The use of joysticks in aircraft flight control systems is well known. In a typical two-axis joystick system, motion of the joystick in the fore-and-aft direction controls the aircraft pitch, and motion of the joystick in the left-right direction controls the aircraft roll. In a related system using a control wheel instead of a joystick, fore-and-aft motion of the control wheel controls pitch, and rotation of the control wheel about its axis controls roll. Using either system, the joystick or control wheel is mechanically coupled by means of cables to the various attitude control subsystems, e.g., to mechanical/hydraulic subsystems for controlling aileron position. In a commercial airliner, safety considerations require two or more separate sets of flight controls, e.g., separate control wheels or joysticks for the pilot and the copilot. In prior flight control systems for commercial aircraft, cables and/or gears have been used to mechanically couple the pilot and copilot flight controls to each other as well as to the various attitude control subsystems.
In modern fly-by-wire flight control systems, the hand movements of the pilot and copilot are first translated into electrical signals by electromechanical transducers, and the electrical signals are in turn used to control the various flight surfaces. In general, one transducer is required for each joystick axis. In a fly-by-wire system, the pilot and copilot flight controls may be either mechanically coupled to one another, or may comprise separate, parallel installations. The use of separate, parallel flight controls for the pilot and copilot presents significant design problems in terms of resolving conflicting signals. Separate, mechanically coupled flight controls can in principle avoid may of such problems. However, prior mechanically coupled systems have been extremely complex and heavy, and have also required separate transducers for each axis and complex interfaces to feel, autopilot and trim systems.